1. Telegram IS centralized.
2. They collect all IPs and domains and ban them on provider level. There is a law which forces internet providers to sync the list of banned resources and prohibit access to them.
Basically they ban hundreds of thousands resources already.
Adding few thousands of nodes may take some time, but:
1. It is possible to do for sure within months
2. When you ban 20-30% of decentralized system it looses 20-30% of users which makes it not that attractive compared to allowed alternatives which are controlled by the government (like vk.com)
Is it because it has centralized servers? What if it was open source and could be hosted on any website?
Then they would have to detect signatures of the protocol and block that, right? At what level, the national level? But the server would be local.
So what does it mean when they say they block Tor for instance?
How can you block something that is run on many domains and servers inside your country?